Abstract

This study examined pituitary sensitivity to exogenous gonadotrophin-releasing hormone (GnRH) in relation to time of year and changes in plasma progesterone, testosterone, luteinising hormone (LH), and follicle-stimulating hormone (FSH) concentrations during the annual cycle of adult and juvenile harbour seals (Phoca vitulina vitulina). There was no significant difference in seasonal changes in reproductive hormone levels between six captive individuals and seals caught from the wild (n = 80). Based on results from both the captive and the wild individuals, the plasma progesterone concentration was elevated in late gestation, whereas it declined at the equivalent time in nonpregnant females. The progesterone concentration was also elevated during the phase of delayed implantation, but there was no difference between pregnant and nonpregnant seals. In captive seals given exogenous GnRH, the LH concentration peaked 10-20 min after injection of GnRH, and the magnitude of the LH response depended upon the dose up to similar to 120 ng.kg(-1). FSH did not respond to GnRH in the time course of the experiment. Pituitary sensitivity to GnRH was greatest in the summer and autumn and lowest in the winter and spring. Seasonal changes in pituitary sensitivity were apparent in both adults and juveniles. There was evidence that three of the four juvenile seals attained puberty during the study. We conclude that the seasonal patterns of pituitary sensitivity and plasma hormone concentration in harbour seals are consistent with a reproductive cycle under photoperiodic control and with the general mechanisms underlying photoperiodic control of seasonal reproduction in higher vertebrates.